In order to prevent contaminatoin of a transparent substrate such as glass, it has been proposed to make the surface as smooth as possible or coat the surface with a protective film such as fluorine-base coating film or the like. Further, to prevent fogging of the transparent substrate surface, a hydrophilic polymer is coated, or a heater is installed in or onto transparent substrate.
Where the contamination of a transparent substrate stems from water drops, an antifogging effect can be obtained by installing a heater, for instance. In this case, however, a power source for the heater is necessary. In addition, a heater which is buried in or installed on the surface of the transparent substrate is liable to reduce the transparency thereof. Coating with a hydrophilic polymer or the like is comparatively simple. However, only a tentative effect is achieved, and the hydrophilic polymer may be easily peeled off by rubbing the transparent substrate surface.
Where contamination stems from other causes than water drops, the above methods are substantially meaningless. Accordingly, it has been proposed to coat the transparent substrate surface with a protective film such as a fluorine-based coating film. However, the adhesion between the transparent substrate and the fluorine-based protective film is weak, and the film is readily separated. In addition, the fluorine-based protective film causes fogging of the transparent substrate due to its opaqueness. There are other protective film materials which can improve the transparency and adhesion. However, these materials cannot permit ready wiping-out of contaminants. Accordingly, it is a practical method to make the surface of the transparent substrate as smooth as possible. Limitations, however, are imposed on the smoothness. After all, there has never been a transparent substrate treated to be contamination free, hydrophobic and/or oil-phobic.